EP3559613B1 - Temperature-resistant fill level measurement device - Google Patents

Description

The invention relates to a radar-based fill level measuring device for measuring a fill level of a product located in a container, wherein the fill level measuring device can also be used at high temperatures prevailing in the container.

In automation technology, in particular in process automation technology, field devices are often used that are used to record and / or influence process variables. To record process variables, sensors are used that are used, for example, in level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc. They record the corresponding process variables, such as level, flow, pressure, temperature, pH value, redox potential or conductivity. A large number of these field devices are manufactured and sold by Endress + Hauser.

For level measurement of products in containers, non-contact measuring methods have become established because they are robust and require little maintenance. Another advantage of non-contact measuring methods is their ability to measure the level quasi continuously, i.e. with a high resolution. For this reason, primarily radar-based measurement methods are used for this. Established measuring principles form the FMCW measuring principle ("Frequency Modulated Continuos Wave") or the so-called pulse transit time method. In both cases, the high-frequency electromagnetic signal is sent in the direction of the product via an antenna which is attached to the container and which is directed into the interior of the container. The frequencies of the emitted electromagnetic waves are in the range of a standardized center frequency. Due to official approval regulations, frequencies at 6 GHz, 26 GHz, or 79 GHz are usually used here. The functional principle of the pulse transit time method is for example in the publication DE 10 2010 063 430 A1 called. For the functional principle of the FMCW measuring principle, refer to the Publication font WO 2012/139852 A1 pointed out. Regardless of the measuring principle, however, higher frequencies are advantageous because the resolution of the level measurement potentially also increases with increasing frequency.

Depending on the area in which the container is used, temperatures of up to 200 ° C can prevail inside the container. In this case, there is a particular challenge in thermally isolating the level measuring device from the inside of the container, since the electronic components of the field device are usually only designed up to a temperature of approximately 80 ° C. The thermal insulation of the electronic components therefore often takes place via a thermally insulating housing neck with high thermal resistance, which is arranged between the antenna and the housing, which is located outside the container. A corresponding level measuring device is for example in the publication DE102009028620 A1 disclosed. The electronic components are located in the housing. Corresponding to this purpose, the housing neck is dimensioned with a certain minimum length and a correspondingly small cross-sectional area, so that the highest possible thermal resistance is established between the antenna and the housing. As further measures for increasing the thermal resistance, it is also known to use a thermally insulating element, for example a ceramic body, in the interior of the housing neck.

Towards high radar frequencies (where high in the context of the invention already means frequencies below 79 GHz), however, for reasons of sufficient signal transmission to the antenna, it is no longer possible to remove the electronic assemblies for high-frequency signal generation from the To arrange antenna: Therefore, in these cases, a (long) housing neck must be avoided.

The invention is therefore based on the object of providing a level measuring device which can be used at high temperatures prevailing in the container.

• Ein Gehäuse,
• eine Antenne, die derart ausgestaltet und am Behälter angeordnet ist, um elektromagnetische Wellen (S_HF) in Richtung des Füllgutes auszusenden und/oder um im Behälter reflektierte elektromagnetische Wellen (EHF) zu empfangen,
• einen Gehäusehals, der zwischen dem Gehäuse und der Antenne angeordnet ist, wobei der Gehäusehals zwischen dem Gehäuse und der Antenne einen vordefinierten thermischen Widerstand (R_th,G) aufweist,
• zumindest eine teilweise im Gehäusehals angeordnete Elektronik-Baugruppe.

The invention solves this problem by means of a radar-based fill level measuring device for measuring a fill level (L) of a product located in a container. It includes:

• A housing
• an antenna which is designed and arranged on the container in order to emit electromagnetic waves (S _HF ) in the direction of the filling material and / or to receive electromagnetic waves (EHF) reflected in the container,
• a housing neck which is arranged between the housing and the antenna, the housing neck having a predefined thermal resistance (R _{th, G} ) between the housing and the antenna,
• at least one electronics assembly partially arranged in the housing neck.

According to the invention, the housing neck is characterized in that the thermal resistance (R _{th, G} ) of the housing neck is dimensioned in such a way that at a temperature (T _B ) near the antenna in the container (or) of at least 200 ° C the temperature (T _E ) on the electronic assembly to a maximum of 80 ° C. Accordingly, the thermal resistance (R _{th, G} ) is to be dimensioned to be less than 15 Kelvin per watt, in particular less than 10 Kelvin per watt. For this purpose, the housing neck is preferably made from a thermally conductive material, in particular from stainless steel, aluminum or copper. Of these materials, copper has the greatest coefficient of thermal conductivity. As a result, a very compact design of the housing neck according to the invention can be achieved. Stainless steel, on the other hand, has the highest mechanical and chemical resistance of the materials mentioned and can therefore be used preferably when the level measuring device is exposed to aggressive ambient conditions.

In the context of the invention, the thermal resistance (R _{th, G} ) is defined as: $${\mathrm{R.}}_{\mathit{th},G}=\frac{1}{{\lambda }_G}\ast \frac{l_G}{{\mathrm{A.}}_G}.$$

I _{G is} the length of the housing neck and A _{G is} its cross-sectional area. λ _G is the coefficient of thermal conductivity of the housing material used with the unit $\left[\frac{\mathrm{W.}}{\mathit{mK}}\right].$

The inventive arrangement of the electronic assembly in the housing neck thus provides a temperature-resistant fill level measuring device which can also be used in the range of high radar frequencies. As a result, better accuracy can be achieved in the fill level measurement than is possible with comparable fill level measuring devices according to the prior art. The temperature resistance of the level measuring device is achieved according to the invention by designing the housing neck with a reduced thermal resistance (R _{th, G} ), contrary to general technical knowledge, and thereby reducing the temperature (T _E ) on the electronics assembly to well below 80 ° C can be. Due to the increased thermal conductivity in the housing neck, the housing neck and antenna components located below the electronics assembly are thermally well coupled to the actual housing, which acts as a heat sink, thereby minimizing the convection currents from the antenna, which may be hot, via the electronics assembly to the housing. A slight increase in the temperature in the actual housing is not critical here.

In addition to the housing material used, the thermal resistance of the housing neck can be influenced according to the above formula by its geometric design, in particular the cross-sectional geometry, the length (I _G ), the mean outside diameter (D _G ) and the mean wall thickness (d _G ). In an advantageous form of the invention, the housing neck therefore preferably has an approximately round cross section with a corresponding outer diameter (D _G ) and a wall thickness (d _G ). The cross-sectional area A _G is measured in this case $${\mathrm{A.}}_G=\pi \left({\mathrm{D.}}_G{d}_G-{\mathrm{<figure-callout\; id="2"\; label="d\; G"\; filenames="imgf0001.png"\; state="\{\{state\}\}">d</figure-callout>}}_G^{}\right).$$

gefertigt ist, eine mittlere minimale Wandstärke (d_G) von 4 mm aufweist. Des Weiteren ist es bezüglich des Außendurchmessers (D_G) vorteilhaft, wenn der Gehäusehals im Fall von Edelstahl einen maximalen mittleren Außendurchmesser (D_G) von 80 mm aufweist. Darüber hinaus ist es möglich, einen erfindungsgemäß geringen thermischen Widerstand (R_th,G) zu erreichen, indem der Gehäusehals für den Fall, dass der Gehäusehals aus Edelstahl gefertigt ist, eine maximale Länge (I_G) von 140 mm aufweist. Im Falle solch einer Auslegung (d_G = 4 mm, D_G = 80 mm, I_G = 140 mm, λ_G = 15 $\frac{W}{\mathit{mK}},$

bei runder Querschnittsfläche A_G) resultiert nach den zuvor genannten Formeln ein thermischer Widerstand (R_th,G) von ca. 9.78 Kelvin pro Watt.In this case, a further development of the fill level measuring device according to the invention provides that the housing neck, in the event that it is made of stainless steel $\left({\mathrm{\lambda }}_{\mathrm{G}}=\mathrm{15th}\frac{\mathrm{W.}}{\mathit{mK}}\right)$

is made, has an average minimum wall thickness (d _G ) of 4 mm. Furthermore, with regard to the outside diameter (D _G ), it is advantageous if the housing neck in the case of stainless steel has a maximum mean outside diameter (D _G ) of 80 mm. In addition, it is possible to achieve a low thermal resistance (R _{th, G ) according to the invention in that the housing neck has a maximum length (I G} ) of 140 mm in the event that the housing neck is made of stainless steel. In the case of such a design (d _G = 4 mm, D _G = 80 mm, I _G = 140 mm, λ _G = 15 $\frac{\mathrm{W.}}{\mathit{mK}},$

with a round cross-sectional area A _G ), according to the aforementioned formulas, a thermal resistance (R _{th, G} ) of approx. 9.78 Kelvin per watt results.

This dimensioning example illustrates an additional advantage: the inventive idea allows the housing neck, and thus the entire fill level measuring device, to be made much more compact than is possible according to the state of the art.

For a further improved thermal decoupling of the electronic assembly from the housing neck and an associated further temperature reduction on the electronic assembly, the housing neck in a development of the invention has an inner wall with a thermally insulating layer. This can be a wall coating. However, it is also possible to use a plastic or ceramic insert. In the case of plastic, in particular PPS (polyphenylene sulfide) can be used. For the same purpose, it is also advantageous if a thermal insulation element is arranged in the housing neck between the electronics assembly and the antenna.

In the simplest case, the connection of the housing neck to the antenna or to the housing is inseparable and is accordingly designed in one piece. Alternatively, however, it is preferably possible that the antenna and / or the housing are / is connected to the housing neck via at least one detachable connection. In this case it is for the purpose of further reducing the thermal resistance (R _{th, G} ) advantageous to provide a thermal paste on the at least one detachable connection.

• Fig. 1: Eine schematische Darstellung eines erfindungsgemäßen Füllstandsmessgerätes an einem Behälter,
• Fig. 2: eine detaillierte Darstellung eines erfindungsgemäßen Füllstandsmessgerätes, und
• Fig. 3: ein Diagramm der Temperatur der elektronischen Komponente in Abhängigkeit der Temperatur im Behälter.

The invention is explained in more detail using the following figures. It shows:

• Fig. 1 : A schematic representation of a fill level measuring device according to the invention on a container,
• Fig. 2 : a detailed representation of a level measuring device according to the invention, and
• Fig. 3 : a diagram of the temperature of the electronic component as a function of the temperature in the container.

For an improved understanding of the invention is in Fig. 1 an arrangement of a radar-based fill level measuring device 1 according to the invention on a container 2 is shown. Inside the container 2 there is a filling material 3, the filling level L of which is to be determined.

To determine the level L, the level measuring device 1 is attached to the container 2 above the product 5 at a previously known installation height h, which can be up to more than 30 m high depending on the size of the container. For this purpose, the level measuring device 1 is arranged by means of a flange connection on the top of the container 2 in such a way that it _{transmits electromagnetic waves S HF} , which are generated by an electronic assembly 131, via an antenna 12 in the direction of the product 3. This can be done according to the FMCW or also the pulse transit time method, for example at a frequency of 79 GHz or higher.

After reflection on the product surface, the level measuring device 1 receives the reflected electromagnetic waves E _HF again via the antenna 12. The transit time measured by the level measuring device 1 is between the transmission and reception of the high-frequency electromagnetic waves S _HF , E _HF depending on the distance a to the product surface. The subsequent calculation of the filling level L from the transit time or the distance a to the filling material surface is carried out by the filling level measuring device 3 with knowledge of its installation height h: L = h - a. The calculation is carried out by a corresponding device electronics 111 of the field device 1, which is encapsulated by a housing 11.

The fill level measuring device 3 according to the invention can, as in FIG Fig. 1 shown, be connected to a higher-level unit 4, for example a process control system, via the device electronics 111 by means of a bus system, for example “PROFIBUS”, “HART” or “Wireless HART”. On the one hand, this can be used to transmit information about the level L in order to control inflows or outflows that may be present on the container 2. However, any information about the operating state of the fill level measuring device 1 can also be communicated.

A process seal 121, for example made of a chemically inert plastic, is arranged inside the antenna 12 in order to seal the fill level measuring device 1 in a fluid-tight manner with respect to the interior of the container 2. In addition to the need for a fluidic seal, however, it is also necessary to protect the level measuring device 1 from temperature influences from the interior of the container 2:

Depending on the intended use, a temperature T _B of up to more than 200 ° C. can prevail in the interior of the container 2, for example due to a chemical reaction which the filling material 3 is currently undergoing. Since the electronic components 111, 131 of the field device 1 are generally only designed up to a temperature T _{E of} up to approx. 80 ° C., the field device 1 comprises a housing neck 13, which is located between the antenna 12, to protect it from the thermal load and the housing 11 is arranged.

In order to achieve the highest possible resolution in the fill level measurement, the electronic assembly 131 for generating high-frequency signals must be arranged close to the antenna 12. The reason for this is that the Coupling of the electromagnetic waves S _HF in the case of high transmit / receive frequencies, for example at 79 GHz, the coupling into the antenna 12 is very lossy at a great distance. The electronics assembly 131 is therefore not arranged in the housing 11 that is further away, but rather in the housing neck 13 located closer to the antenna 12. In order to protect the electronic assembly 131 from any thermal load from the interior of the container 2, the housing neck 13 according to the invention has such a low thermal resistance R _{th, G} that at a temperature of at least 200 ° C in the container 2 ( or at the location of the antenna 12) the temperature T _{E of} the electronic component 131 rises to a maximum of 80.degree.

A detailed cross-sectional view of the fill level measuring device 1 according to the invention is shown in FIG Fig. 2 shown. In this illustration, the influencing variables of the thermal resistance R _{th, G} can be seen in more detail: In addition to the material from which the housing neck 13 is made, these are above all the geometric dimensions of the housing neck 13: the length I _G , the outer diameter D _G and the Wall thickness d _{G of} the housing neck 13 (in the illustration shown, the housing neck 13 has an approximately round cross section).

λ_G verringert sich die Temperatur T_E an der Elektronik-Baugruppe 131 von ca. 83 °C auf ungefähr 63 °C, welches nunmehr eine unkritische Temperatur für die Elektronik-Baugruppe 131 darstellt. Diesen Simulationswerten liegt die Annahme zugrunde, dass der Gehäusehals 13 bei einer Wandstärke d_G von 5.6 mm zur thermischen Entkopplung der elektronischen Baugruppe 131 zusätzlichen einen 1 mm dicken thermisch isolierenden Einsatz 132 aus PPS an der Innenwand des Gehäusehalses 13 aufweist (siehe Fig. 2). Wie in Fig. 2 dargestellt, wäre es zu diesem Zweck außerdem möglich, im Gehäusehals 13 ein thermisches Isolationselement 135 (bspw. eine entsprechend ausgestaltete Keramik mit einem hohen thermischem Widerstand) zwischen der elektronischen Komponente 131 und der Antenne 12 anzuordnen.For example, how a change in the wall thickness d _G affects the thermal resistance R _{th, G of} the housing neck 13 is shown in FIG Fig. 3 In the case of a temperature of 200 ° C in container 2 and an increase in wall thickness d _G from 2 mm to 5.6 mm (this corresponds to a reduction in thermal resistance R _{th, G} from approx. 19.04 Kelvin / watt to approx. 7.13 Kelvin / watt, the other measurements remain unchanged at D _G = 80 mm, I _G = 140 mm, ${\mathrm{\lambda }}_{\mathrm{G}}=\mathrm{15th}\frac{\mathrm{W.}}{\mathit{mK}})$

λ _G , the temperature T _E at the electronics assembly 131 decreases from approximately 83 ° C. to approximately 63 ° C., which now represents a non-critical temperature for the electronics assembly 131. These simulation values are based on the assumption that the housing neck 13 with a wall thickness d _G of 5.6 mm for thermal decoupling of the electronic assembly 131 also has a 1 mm thick thermally insulating insert 132 made of PPS on the inner wall of the housing neck 13 (see Fig. 2 ). As in Fig. 2 For this purpose, it would also be possible to arrange a thermal insulation element 135 (for example a correspondingly configured ceramic with a high thermal resistance) in the housing neck 13 between the electronic component 131 and the antenna 12.

From the diagram in Fig. 3 is shown, it can also be seen that a slight increase in the temperature of the device electronics 111 in the housing 11 is associated _{with the reduction in the thermal resistance R th, G.} In the context of the invention, however, this can be accepted, since the temperature of the device electronics 111 continues to remain well below 80.degree. Rather, the effect of the invention is that the electronic component 131 located in the housing neck 13 is also thermally decoupled from the container 2 from the container in such a way that the temperature T _E at the electronic component 131 in the housing neck 13 even at a temperature of 200 ° C. in the container 2 remains below 80 ° C.

List of reference symbols

1

Level measuring device

2

container

3

Filling material

4th

Parent unit

11

casing

12th

antenna

13th

Housing neck

111

Device electronics

121

Process seal

131

Electronics assembly

132

Thermally insulating layer

133

Detachable connection

134

Detachable connection

135

Thermal insulation element

AG

Cross-sectional area of the housing neck

a

Distance to the product

DG

Outside diameter of the housing neck

dG

Wall thickness of the housing neck

EHF

Electromagnetic waves

H

Installation height of the level measuring device

L.

Level

IG

Length of the case neck

Rth, G

Thermal resistance of the housing neck

TB

Temperature in the container

TE

Temperature on the electronic assembly

λG

Thermal conductivity coefficient of the housing material

Claims (11)

1. Radar-based level transmitter, designed to measure a level (L) of a medium (3) located in a vessel (2), comprising

   - a housing (11),

   - an antenna (12), which is designed and arranged on the vessel (2) in such a way to transmit electromagnetic waves (S_HF) in the direction of the medium (3) and/or to receive electromagnetic waves (E_HF) reflected in the vessel (2),

   - a housing neck (13), which is arranged between the housing (11) and the antenna (12), wherein the housing neck (13) has a predefined thermal resistance (R_th,G) between the housing (11) and the antenna (12),

   - at least an electronics assembly (131) partially arranged in the housing neck (13),

   characterized in that
   the thermal resistance (R_th,G) of the housing neck (13) is sized in such a way to be less than 15 Kelvin per Watt such that at a temperature (T_B) in the vessel (2) of at least 200 °C, the temperature (T_E) at the electronics assembly (131) is at maximum 80 °C.
2. Level transmitter as claimed in Claim 1,
   characterized in that
   the thermal resistance (R_th,G) of the housing neck (13) is sized to be less than 10 Kelvin per Watt.
3. Level transmitter as claimed in Claim 1 or 2,
   characterized in that
   the housing neck (13) is made from a thermally conductive material, particularly stainless steel, aluminum or copper.
4. Level transmitter as claimed in at least one of the previous claims,
   characterized in that
   the housing neck (13) has a cross-section that is approximately round.
5. Level transmitter as claimed in Claim 4,
   characterized in that
   the housing neck (13) has an average minimum wall thickness (d_G) of 4 mm in the event that the housing neck (13) is made from stainless steel.
6. Level transmitter as claimed in Claim 4 or 5,
   characterized in that
   the housing neck (13) has a maximum average outer diameter (D_G) of 80 mm in the event that the housing neck (13) is made of stainless steel.
7. Level transmitter as claimed in one or more of the Claims 4 to 6,
   characterized in that
   the housing neck (13) has a maximum length (I_G) of 140 mm in the event that the housing neck (13) is made of stainless steel.
8. Level transmitter as claimed in at least one of the previous claims,
   characterized in that
   the housing neck (13) has an interior wall with an insulating thermal layer (132), particularly made of polyphenylene sulfide.
9. Level transmitter as claimed in at least one of the previous claims,
   characterized in that
   the antenna (12) and/or the housing (11) is/are connected to the housing neck (13) via at least a detachable connection (133, 134).
10. Level transmitter as claimed in Claim 9,
    characterized in that
    a heat transfer paste is provided at the at least one detachable connection (133, 134).
11. Level transmitter as claimed in at least one of the previous claims,
    characterized in that
    a thermal isolation element (135) is arranged in the housing neck (13) between the electronics assembly (131) and the antenna (12).

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